There is conventionally available a sealed contact device B which includes, as shown in FIGS. 8A, 8B, 9A, 9B and 10A through 10C, a hollow box-shaped case 4 and an inner block 1 arranged within the case 4, the inner block 1 having an electromagnet block 2 and a contact block 3 combined together (see, e.g., Japanese Patent Application Publication No. H11-238443). In the description given below, an up-down direction, a left-right direction and a front-rear direction orthogonal to the up-down direction and the left-right direction will be defined on the basis of the directions shown in FIG. 8A.
The electromagnet block 2 includes a hollow cylindrical coil bobbin 21 made of an insulating material and wound with an exciting coil 22, a pair of coil terminals 23 connected to the opposite end portions of the exciting coil 22, a stationary iron core 24 fixed to the inside of the coil bobbin 21 and magnetized by the energized exciting coil 22, a movable iron core 25 arranged within the coil bobbin 21 in an axially opposing relationship with the stationary iron core 24 so that, upon energizing and de-energizing the exciting coil 22, the movable iron core 25 can be attracted by the stationary iron core 24 and axially moved within the coil bobbin 21, a yoke 26 made of a magnetic material and arranged to surround the coil bobbin 21, and a return spring 27 arranged within the coil bobbin 21 to bias the movable iron core 25 downwards.
The contact block 3 includes a sealing container 31 formed of an insulating material and having a hollow box-shape with an open lower surface, a pair of substantially cylindrical columnar fixed terminals 33 arranged to extend through an upper surface of the sealing container 31 and provided with fixed contact points 32 on its lower surface, a movable contact member 135 arranged within the sealing container 31 and provided with movable contact points 34 for moving toward and away from the fixed contact points 32, a pressure contact spring 36 kept in contact with a lower surface of the movable contact member 135 to bias the movable contact member 135 toward the fixed contact points 32, and a shaft 37 coupled with the movable contact member 135 at its upper end and connected to the movable iron core 25 at its lower end to move together with the movable iron core 25.
The coil bobbin 21 is formed of a resin material and has a hollow cylindrical shape. The coil bobbin 21 includes upper and lower flange portions 21a and 21b and a cylinder portion 21c. The exciting coil 22 is wound around the outer circumference of the cylinder portion 21c. The inner diameter of a lower extension of the cylinder portion 21c is greater than the inner diameter of an upper extension thereof.
As shown in FIGS. 10B and 10C, the exciting coil 22 is connected at its opposite ends to a pair of terminal portions 121 provided in the upper flange portion 21a of the coil bobbin 21. Then, the exciting coil 22 is connected to the coil terminals 23 through lead lines 122 extending from the terminal portions 121, respectively.
Each of the coil terminals 23 includes a base portion 23a made of an electrically conductive material such as copper and connected to the lead lines 122 by soldering or other methods, and a terminal portion 23b arranged to extend substantially vertically from the base portion 23a. 
As shown in FIG. 10B, the yoke 26 includes a substantially rectangular first yoke plate 26A arranged at the upper end side of the coil bobbin 21, a substantially rectangular second yoke plate 26B arranged at the lower end side of the coil bobbin 21 and a pair of third yoke plates 26C arranged to extend upwards from the left and right end portions of the second yoke plate 26B and connected to the first yoke plate 26A.
A recessed portion 26a is formed substantially at the center of an upper surface of the first yoke plate 26A. An insertion hole 26c is defined substantially at the center of the recessed portion 26a. A closed-bottom cylinder member 28 with an upper flange portion 28a is inserted into the insertion hole 26c. The upper flange portion 28a is jointed to the recessed portion 26a. The movable iron core 25 is formed from a magnetic material into a substantially cylindrical columnar shape and is arranged within the lower extension of the cylinder portion 28b of the cylinder member 28. The stationary iron core 24 is formed from a magnetic material into a substantially cylindrical columnar shape and is inserted into the cylinder portion 28b in an opposing relationship with the movable iron core 25.
A metal-made cap member 45 is arranged on the upper surface of the first yoke plate 26A. The cap member 45 includes a peripheral edge portion fixed to the first yoke plate 26A and a raised portion 45a formed substantially at the center thereof to define a space for accommodating the upper flange portion 24a of the stationary iron core 24. Removal of the stationary iron core 24 is prevented by the cap member 45.
A cylindrical bush 26D made of a magnetic material is fitted to a gap portion between the lower inner circumferential surface of the coil bobbin 21 and the outer circumferential surface of the cylinder member 28. The bush 26D makes up a magnetic circuit in cooperation with the yoke 26, the stationary iron core 24 and the movable iron core 25.
The return spring 27 extends through an axial insertion hole 24b of the stationary iron core 24. The return spring 27 makes contact with the upper surface of the movable iron core 25 at its lower end and with the lower surface of the cap member 45 at its upper end. The return spring 27 is kept compressed between the movable iron core 25 and the cap member 45, thereby resiliently biasing the movable iron core 25 downwards.
The shaft 37 is formed of a non-magnetic material to have a vertically elongated bar shape. The shaft 37 extends through an insertion hole 45b of the cap member 45 defined substantially at the center of the raised portion 45a and then through the return spring 27. The shaft 37 includes a thread portion 37b formed in the lower extension thereof. The thread portion 37b is threadedly coupled with, and connected to, an axial thread hole 25a of the movable iron core 25.
The movable contact member 135 includes a substantially rectangular body portion 135a having an insertion hole 135d defined substantially at the center thereof. The shaft 37 is inserted into the insertion hole 135d. Movement of the movable contact member 135 toward the fixed contact points 32 is restrained by a flange-shaped restraint portion 37a formed at the upper end of the shaft 37. As can be seen in FIG. 11A, the movable contact points 34 are fixed to the left and right end portions of the body portion 135a. Substantially rectangular protrusions 135b and 135c protrude from the longitudinal sides of the body portion 135a. The protrusions 135b and 135c are substantially in point symmetry with respect to the insertion hole 135d. The protrusions 135b and 135c are formed to have the same width a5 and the same protruding length b5.
The fixed terminals 33 are formed from an electrically conductive material such as copper into a substantially cylindrical columnar shape. Each of the fixed terminals 33 includes a flange portion 33a formed at the upper end thereof. The fixed contact points 32 are fixed to the lower surfaces of the fixed terminals 33 in an opposing relationship with the movable contact points 34. Each of the fixed terminals 33 has a thread hole 33b axially extending from the upper surface thereof. Thread portions of an external load device (not shown) are threadedly coupled to the thread holes 33b of the fixed terminals 33.
The sealing container 31 is formed from a heat-resistant material such as ceramics into a hollow box shape with an open lower surface. Two through-holes 31a through which the fixed terminals 33 pass are formed side by side on the upper surface of the sealing container 31. The fixed terminals 33 are inserted into the through-holes 31a with the flange portions 33a thereof positioned above the upper surface of the sealing container 31 and are jointed to the sealing container 31 by soldering. As shown in FIG. 10A, one end of a flange member 38 is jointed to the peripheral edge of an opening of the sealing container 31 by soldering. The other end of the flange member 38 is jointed to the first yoke plate 26A by soldering, whereby the sealing container 31 is sealed.
In the opening of the sealing container 31, there is provided an insulating member 39 for isolating an arc generated between the fixed contact points 32 and the movable contact points 34 from the joint portion of the sealing container 31 and the flange member 38.
The insulating member 39 is formed from an insulating material such as ceramics or a synthetic resin into a substantially hollow cuboid shape with an open upper surface. The insulating member 39 includes a rectangular rim 39a formed substantially at the center of the lower surface thereof. The raised portion 45a of the cap member 45 is fitted to a recessed portion defined within the rectangular rim 39a. The upper end of a peripheral wall of the insulating member 39 makes contact with the inner surface of a peripheral wall of the sealing container 31, thereby isolating the joint portion of the sealing container 31 and the flange member 38 from the contact portions including the fixed contact points 32 and the movable contact points 34.
The insulating member 39 includes a circular rim 39c formed substantially at the center of the inner bottom surface thereof. The inner diameter of the circular rim 39c is substantially equal to the inner diameter of the pressure contact spring 36. An insertion hole 39b through which the shaft 37 extends is formed substantially at the center of the circular rim 39c. The lower end of the pressure contact spring 36 through which the shaft 37 extends is fitted to a recessed portion defined within the circular rim 39c, thereby preventing misalignment of the pressure contact spring 36.
The pressure contact spring 36 makes contact with the lower surface of the movable contact member 135 at its upper end and remains compressed between the insulating member 39 and the movable contact member 135. Thus, the pressure contact spring 36 resiliently biases the movable contact member 135 toward the fixed contact points 32.
The case 4 is formed from a resin material into a substantially rectangular box shape. The case 4 includes a hollow box-shaped case body 41 with an open upper surface and a hollow box-shaped cover 42 arranged to cover an opening of the case body 41.
As shown in FIG. 10C, the case body 41 includes ear portions 141 formed at the front ends of the left and right side walls thereof. The ear portions 141 have insertion holes 141a used in attaching the sealed contact device B to an installation surface with screws. The case body 41 has a stepped portion 41a formed in the peripheral edge of the upper opening thereof. The outer dimension of the upper end extension of the case body 41 is smaller than the outer dimension of the lower extension thereof. A pair of slits 41b for insertion of the terminal portions 23b of the coil terminals 23 is formed in the front wall of the case body 41 above the stepped portion 41a. In the rear wall of the case body 41 above the stepped portion 41a, a pair of recessed portions 41c is arranged side by side along the left-right direction.
The cover 42 is formed into a hollow box shape with an open lower surface. A pair of protrusions 42a is formed on the rear surface of the cover 42. The protrusions 42a are fitted to the recessed portions 41c of the case body 41 when the cover 42 is mounted to the case body 41. A partition portion 42c for substantially bisecting the upper surface of the cover 42 into left and right areas is formed on the upper surface of the cover 42. A pair of insertion holes 42b for insertion of the fixed terminals 33 is formed on the upper portion of the cover 42 bisected by the partition portion 42c. 
When the inner block 1 including the electromagnet block 2 and the contact block 3 is put into the case 4, a substantially rectangular lower cushion rubber 43 is interposed between the lower flange portion 21b of the coil bobbin 21 and the bottom surface of the case body 41 and an upper cushion rubber 44 having insertion holes 44a for insertion of the flange portions 33a of the fixed terminals 33 is interposed between the sealing container 31 and the cover 42.
In the conventional sealed contact device B configured as above, the return spring 27 has a spring constant higher than that of the pressure contact spring 36. Therefore, the movable iron core 25 is slid downwards by the biasing force of the return spring 27 and, concurrently, the shaft 37 is moved downwards. Since the movable contact member 135 is moved downwards together with the restraint portion 37a of the shaft 37, the movable contact points 34 are initially kept spaced apart from the fixed contact points 32.
If the exciting coil 22 is energized, the movable iron core 25 is attracted by the stationary iron core 24 and moved upwards. Thus, the shaft 37 connected to the movable iron core 25 is also moved upwards. As a result, the restraint portion 37a of the shaft 37 is moved toward the fixed contact points 32, and the movable contact member 135 is also moved toward the fixed contact points 32 by the biasing force of the pressure contact spring 36. Accordingly, the movable contact points 34 fixed to the movable contact member 135 are brought into contact with, and electrically connected to, the fixed contact points 32.
If the exciting coil 22 is de-energized, the movable iron core 25 is slid downwards by the biasing force of the return spring 27. Accordingly, the shaft 37 is also moved downwards. As a result, the restraint portion 37a is moved downwards together with the movable contact member 135, whereby the fixed contact points 32 and the movable contact points 34 are spaced apart from each other and electrically interrupted.
In the conventional sealed contact device B described above, the pressure contact spring 36 is kept compressed. Therefore, if the pressure contact spring 36 is extended to cause the movable contact member 135 to slide toward the fixed contact points 32, the movable contact member 135 is rotated clockwise as illustrated in FIG. 11B by the torque of the pressure contact spring 36 acting in the direction (clockwise direction) opposite to the winding direction (counterclockwise direction) thereof. If the pressure contact spring 36 is retracted to move the movable contact member 135 away from the fixed contact points 32, the movable contact member 135 is rotated counterclockwise by the torque of the pressure contact spring 36 acting in the same direction as the winding direction thereof.
Consequently, the movable contact member 135 makes sliding movement in a state that two diagonal points of the protrusions 135b and 135c remaining in point symmetry with respect to the insertion hole 135d are brought into contact with, and pressed against, the inner surfaces of the sealing container 31. This leads to an increased friction force and hinders smooth movement of the movable contact member 135, which may possibly impair the reliability of a switching action between the contact points.
In general, if the contact points are electrically connected to each other, electric currents flow in the opposite directions on the surfaces of the fixed contact points 32 and on the surfaces of the movable contact points 34 opposing to the fixed contact points 32. This generates an electromagnetic repulsion force acting to move the movable contact points 34 away from the fixed contact points 32.
If the movable contact member 135 is tilted by, e.g., an unbalanced biasing force applied from one end of the pressure contact spring 36 and if the centers of the movable contact points 34 make contact with the off-centered areas of the fixed contact points 32, the electromagnetic repulsion force mentioned above acts on the movable contact member 135 as rotation torque. When the contact points are electrically connected or when the intensity of an electric current flowing between the contact points is changed sharply, the movable contact member 135 is continuously affected by the variations of the rotation torque and is vibrated about the connection portion thereof connected to the shaft 37. Abnormal noises may possibly be generated by the vibration of the movable contact member 135.